The present invention relates to a semiconductor device, which has improved a heat dissipation property of a package for containing semiconductor dies.
Semiconductor devices used in microwave frequency is so constructed that, for example, semiconductor dies for amplifying radio frequency signals and various circuit boards connected with the semi-conductor dies are arranged in a package. A package is composed of, a bottom plate for fixing semiconductor dies and circuit boards, a side wall provided on the periphery of the bottom plate so as to surround the semiconductor dies and circuit boards, and a lid for covering upper opening formed by the side wall.
By the way, usually a plurality of power amplifying semiconductor dies is used in a semiconductor device for power use. Therefore, a high mechanical strength is required for a package for power use and the size becomes large because a plurality of semiconductor dies and circuit boards to be connected with them should be contained therein.
Further, packages used for semiconductor device for power use, need to discharge heat generated by semiconductor dies. For this reason, such metal having high heat conductivity and relatively low price as copper is generally used. However, copper is soft and ductile, and packages having enough mechanical strength for power use cannot be obtained.
Therefore material having a so called Copper-Molybdenum-Copper structure is used, in which molybdenum or tungsten plate harder than copper is interposed between two copper plates as support metal, to improve mechanical strength in a package for power use with high heat generation. Hereinafter, material having such structure is called as a CMC structure.
Here, a conventional semiconductor device using a CMC structure package is explained referring to FIG. 1A and FIG. 1B. FIG. 1A is a top view showing the package with its lid removed and FIG. 1B is a cross section showing a section along the line b-b in FIG. 1A seen along an arrow direction. In FIG. 1A and FIG. 1B, the corresponding parts are assigned with same symbols.
A package 32 is mounted on a metal plate 31. The package 32 is composed of a bottom plate 32a, a sidewall 32b, and a lid (not illustrated) for covering the opening at the upper portion of the sidewall 32b. The bottom plate 32a has the CMC structure, formed in a rectangular-shape as a whole, for example. Namely, the bottom plate 32a has a laminated structure, in which a metal plate M harder than copper is interposed between two copper plates C1 and C2 as shown in FIG. 1B. The metal plate M is, for example, molybdenum Mo or tungsten W. On opposing edges of the bottom plate 32a, two screw holes 33 for fixing the package 32 on the plate 31 by screw are provided respectively.
A sidewall 32b is provided on the bottom plate 32a. The major portion of the sidewall 32b is made of metal and is formed as a whole rectangular frame in shape. At center portions of two opposing sides of the sidewall 32b, an input metal lead 34a and an output metal lead 34b are provided, which penetrates through the sidewall 32b. At portions surrounding the metal lead 34a, 34b, insulator 35a, 35b such as ceramic member are provided in order to prevent the metal lead 34a, 34b from contacting with the metal portion of the sidewall 32b. One end of the metal lead 34a, 34b projects inside the sidewall 32b and the insulator 35a, 35b are provided under the projection portion a1, b1.
A semiconductor element 36 for microwave power amplification is mounted on a central portion of the bottom plate 32a surrounded by the sidewall 32b. The semiconductor element 36 for microwave power amplification contains, for example, four semiconductor dies 36a to 36d. On the input metal lead 34a side of the bottom plate 32a, a power divider circuit 37 including an input matching circuit formed on the surface of the dielectric plate is located. On the output metal lead 34b side of the bottom plate 32a, a power combiner circuit 38 including an output matching circuit is located. The semiconductor dies 36a to 36d are electrically connected with the power divider circuit 37 and with the power combiner circuit 38 by wires W. Also, the input metal lead 34a is electrically connected with the power divider circuit 37 and the power combiner circuit is electrically connected with the output metal circuit 34b by wires W.
In the device thus constructed, a radio frequency big power signal is supplied through the input metal lead 34a, for example. The radio frequency signal is divided into four by the power divider circuit 37 and each of divided t signal is amplified in its power by semiconductor dies 36a to 36d. Then, the divided signals were combined by the power combiner circuit 38 and outputted through the output metal lead 34b. 
When amplifying a radio frequency signal having a large power, heat is generated from the semiconductor 36a to 36d. A part of heat generated is dissipated through the bottom plate 32a from the lower surface to the metal plate 31 below. At this time, heat is dissipated through the junction of the bottom plate 32a and the metal plate 31 from the lower surface of the bottom plate 32a to the metal plate 31 with a spread angle of more than 45°. Further, a part of the heat generated in the semiconductor dies 36a to 36d is radiated from the upper surface of the semiconductor dies 36a to 36d upward in the figure by a black body radiation. Here, major portion of the dissipated heat is a downward radiation. The semiconductor device containing semiconductor element and associated circuits in a package as mentioned above is disclosed in Japanese published patent application 2001-257234.
In the conventional semiconductor device, the thermal expansion coefficient of the package becomes higher than that of the dielectric plate such as a ceramic plate, on which a power divider circuit or the power combiner circuit is formed, if the package is formed with copper itself. Therefore, the ceramic plate may have a crack by thermal shrinkage afterwards, if the power divider circuit or the power combiner circuit is soldered on the bottom plate of the package.
On the other hand, the crack generated in the ceramic plate can be avoided, if the bottom plate of the package is formed as a CMC structure material, because a heat expansion coefficient of molybdenum is close to that of the ceramic plate composing the power divider circuit or the power combiner circuit. However, heat dissipation ability is decreased because thermal conductivity of molybdenum is as low as about 40% of that of copper and the heat conductivity of the bottom plate including the junction of molybdenum and copper is less than a half of that of copper.